The present invention relates to a ferromagnetic material having a wide range of application in technical fields, and magnetic apparatuses including high-density magnetic recording apparatuses and magnetic sensors employing such a ferromagnetic material.
A ferromagnetic material is a substance having spontaneous magnetization, i.e., a substance having a finite magnetization intensity. Sometimes, a ferromagnetic material in bulk does not display any finite magnetization intensity. In such a state, the interior of the ferromagnetic material is divided into a plurality of regions, each of the regions displays magnetization of a finite intensity, and the directions of magnetization of the regions are different from each other. A small region in which spontaneous magnetization has a fixed direction is called a magnetic domain.
A ferromagnetic material is applied widely to various magnetic devices including various magnetic recording systems and magnetic sensors. Efforts have been made for the development of various ferromagnetic materials suitable for different purposes. In the field of magnetic recording, in particular, the reduction in size of magnetic domains and the realization of recording of a minimum unit by a smallest possible number of magnetic domains are important problems. Although a plurality of magnetic domains serve as a recording unit in current magnetic recording, it is desirable to use a single magnetic domain as a recording unit when all is said and done, and it is desirable to reduce the size of magnetic domains each for a recording unit.
A method of making a ferromagnetic material having small magnetic domains employing electron beam lithography is disclosed in, for example, Journal of Applied Physics, Vol. 76, pp. 6673-6675 (1994). This method forms a region of several tens nanometer square of magnetic atoms on a nonmagnetic substrate, i.e., a material not displaying ferromagnetism. It is reported in this paper that the region displays ferromagnetism in a single magnetic domain in some cases. Magnetic atoms are atoms which display ferromagnetism in single bulk, such as those of 3d transition metals including Cr, Mn, Fe, Co and Ni, and lanthanides including Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm.
A region of a ferromagnetic material of a size on the order of the foregoing size can be formed by depositing the ferromagnetic material on a nonmagnetic substrate with the probe of a scanning tunneling microscope (STM) as mentioned in, for example, Journal of Applied Physics, Vol. 76, pp. 6656-6660 (1994).
Further miniaturization can not be successfully achieved simply by reducing the size of the region of a magnetic material by the direct application of the foregoing two methods. Reason for it will be explained in terms of the Stoner""s model which is used for explaining ordinary bulky ferromagnetic materials, such as Fe, Co and Ni. As mentioned in Tokyo Daigaku Bussei Kenkyu-jo, xe2x80x9cBussei Kagaku Jitenxe2x80x9d, pp. 198-200, Tokyo Shoseki (1996), the Stoner""s model expresses a condition for displaying ferromagnetism (Stoner condition) by Uxc3x97D(Ef) greater than 1, where U is electron correlation energy or energy of Coulomb repulsion between electrons, and D(Ef) is electronic density of states at Fermi level. A substance must have a very large density of states D(Ef) on a Fermi surface to be ferromagnetic. However, if a minute atomic cluster system or a fine atom wire system is formed by a ferromagnetic material meeting the Stoner condition, the density of states D(Ef) is reduced greatly by a finite size effect and, consequently, the Stoner condition cannot be met and it is highly possible that the spontaneous magnetization of the system disappears.
Accordingly, a novel idea entirely different from conventional ideas is necessary to realize a ferromagnetic material which makes possible a further smaller single magnetic domain.
Accordingly, it is an object of the present invention to provide a ferromagnetic material from which spontaneous magnetization does not disappear even if the magnetic domain is further miniaturized.
A second object of the present invention is to provide a magnetic head capable of controlling a magnetic field created by a minute magnetic head comprising an atomic group or a fine atom wire formed on a surface of a solid by applying voltage to the surface of the solid as contrasted with a recording system which supplies a current to an electromagnetic induction magnetic head.
A third object of the present invention is to provide a magnetoresistance effect element including a fine wire having a function similar to that of a magnetoresistance device in a spin valve or a magnetic multilayer film (or an artificial super lattice of magnetic/nonmagnetic metals) or a function analogous to the magnetoresistance effect of ferromagnetic tunnel junction.
To solve the foregoing problems, the present invention utilizes a fact that the atomic arrangement and the electronic state of a surface of a solid, and an atom or an atomic group (including molecules) on a surface of a solid, are different from those of a bulk, i.e., a macroscopic object. Ferromagnetism is displayed by properly arranging atoms on a surface of a substrate. It is a feature of the present invention that ferromagnetism is displayed by using only nonmagnetic atoms. Nonmagnetic atoms are those excluding the previously defined magnetic atoms and atoms of rare gases (He, Ne, Ar, Kr, Xe and Rn).
As mentioned above, according to the Stoner""s model, the condition for displaying ferromagnetism, i.e., the Stoner condition, is expressed by Uxc3x97D (Ef) greater than 1, where U is electron correlation energy or energy of Coulomb repulsion between electrons, and D(Ef) is electronic density of states at Fermi level. Therefore, even substances which are nonmagnetic in a bulk state can be made to display ferromagnetism if the Stoner condition: Uxc3x97D (Ef) greater than 1 can be met by properly arranging atoms on a surface of a substrate. For example, the appearance of spontaneous magnetization at an end of a graphite ribbon is theoretically predicted in, for example, Journal of Physical Society of Japan, Vol. 65, pp. 1920-1923 (1996). However, this structure has not been realized as yet.